1. Field of the Invention
The present invention relates to a method for depolymerization of polytetrahydrofuran derivatives, and particularly to the use of xcex2-zeolite as a catalyst for converting polytetrahydrofuran derivatives to tetrahydrofuran.
2. Description of the Related Art
Polytetramethylene ether glycols (hereinafter as PTMEG), widely used as the polyol component with isocyanates to make thermoplastic polyurethanes, are obtained by the alcoholysis of polytetramethylene ether acetates (hereinafter as PTMEA).
In addition, PTMEA is prepared by polymerizing anhydrous tetrahydrofuran (hereinafter as THF) in the presence of acetic anhydride, acetic acid, and acid catalysts such as fluosulfonic acid (HSO3F). In industrial production, if the catalyst exhibits-abnormal activity, or reaction system used to prepare the PTMEA is changed, it may obtain a mixing product, giving off-specification fractions composed of polymers of different molecular weight ranges. Usually, those off-specification materials are continuously alcoholyzed, separated, and purified to obtain PTMEG, which is then depolymerized to a monomer, namely to THF, and recovered. This recovery also arises whenever uncontrolled events during the synthesis lead to polymers which do not conform to the required narrow specification. Therefore, if the off-specification materials containing PTMEA are recovered directly in the preparation process, significant costs and downstream operations can be conserved.
The following prior art is directed to the process for the depolymerization of PTMEG, including:
U.S. Pat. No. 4,115,408 describes a process for depolymerizing PTMEG to THF, in which the effluent containing dissolved PTMEG is heated with sulfuric acid at the temperature high than 150xc2x0 C. This process has two disadvantages in particular. First, the use of relatively concentrated acid at the temperature high than 150xc2x0 C. necessarily entails substantial corrosion problems, and, secondly, the dilute aqueous sulfuric acid obtained after the depolymerization must be neutralized before it can be discharged into the sewer, thereby imposing a substantial salt load on the sewer.
U.S. Pat. No. 4,363,924 discloses the use of a bleaching earth as a catalyst. However, the activity level and life of this catalyst can be improved.
Japanese Patent No. 60-109584 discloses the use of heteropoly acid as a catalyst. Due to its high cost, use of this catalyst carries distinct economic disadvantages.
Japanese Patent No. 62-257931 discloses the use of non-crystalline SiO2xe2x80x94Al2O3 as a catalyst. However, the activity level and life of this catalyst can be improved.
WO 95/02625 discloses the use of metal perfluoroalkylsulfonates, e.g. (CF3SO3)3Y, as a catalyst in the presence of an accelerator. Due to the high cost, use of this catalyst carries distinct economic disadvantages.
German Patent No. DE 4410685 discloses a process for depolymerizing PTMEG, which comprises heating PTMEG in the presence of kaolin, amorphous silica, and/or X-zeolite. However, the activity level and life of these catalysts can be improved.
Japanese Patent No. 11-269262 uses the mixture of ZrO2 and SiO2 as a catalyst. However, the activity level of this catalyst can be improved.
All the above-cited catalyst systems are used in the depolymerization of PTMEG. No prior art teaches the depolymerization of PTMEA in the industrial process. Further, the yield and the costs of the catalysts employed in the prior art can also be improved. Therefore, there is still a need for development of a catalyst system for depolymerization of PTMEG and/or PTMEA in a more efficient manner.
The present invention, therefore, discloses the use of xcex2-zeolite showing higher activity levels and longer life as a catalyst than those disclosed in the prior art.
The main object of the invention is to provide a method for depolymerization of a mixture comprising polytetrahydrofuran derivatives. The method comprises heating the mixture in the presence of a xcex2-zeolite catalyst at 100-250xc2x0 C. to obtain THF, whereby the THF monomers can be recovered from the waste materials in the preparation of PTMEG.
In one preferred example, the polytetrahydrofuran derivatives comprise polytetramethylene ether glycols or polytetramethylene ether acetates.
In another preferred example, the mixture comprising polytetrahydrofuran derivatives of the invention further comprises by-products or impurities such as water, sodium chloride, and sodium acetate during the process of separation and purification.
In another preferred example, the depolymerization method of the invention is carried out at 130-210xc2x0 C.
The equations of reacting PTMEA with or without water to obtain THF are as follows:
CH3COOxe2x80x94(C4H8O)nxe2x80x94OCCH3xe2x86x92(nxe2x88x921)C4H8O+CH3COOxe2x80x94C4H8Oxe2x80x94OCCH3xe2x80x83xe2x80x83(1)
CH3COOxe2x80x94(C4H8O)nxe2x80x94OCCH3+H2Oxe2x86x92nC4H8O+2CH3COOHxe2x80x83xe2x80x83(2)
In addition, the equation of converting PTMEG to THF is as follows:
HOxe2x80x94(C4H8O)nxe2x80x94Hxe2x86x92nC4H8O+H2Oxe2x80x83xe2x80x83(3)
As used herein, the term xe2x80x9cpolytetrahydrofuran derivativesxe2x80x9d includes PTMEG and PTMEA. That is, both reactants can be reacted in one process or in separate processes, according to the present invention to obtain THF monomers for recovery.
Distinct from the X-zeolite used in the process disclosed in DE 4410685, a xcex2-zeolite is employed as a catalyst according to the present invention. xcex2-zeolite was first synthesized in 1967, but its structure was not published until 1988. It is the only high silica zeolite having a full three-dimensional network of 12-membered rings and is somewhat like the faujasite structure. It probably has the most complex zeolite structure yet determined. Therefore, xcex2-zeolite is different from other known zeolites in terms of structure and physical-chemical properties. Thus, xcex2-zeolite is first used in the depolymerization of polytetrahydrofuran derivatives in the context of the invention.
In accordance with the invention, the method can be carried out in the environment of an acidic catalyst.
As used herein, the term xe2x80x9cwaste materialsxe2x80x9d includes: (1) off-specification fractions composed of PTMEG and/or PTMEA with different molecular weight ranges (e.g. molecular weight higher than the specification); (2) the PTMEG of small molecular weight cut off from the prepared PTMEG fractions; or (3) by-products or impurities such as water, sodium chloride, and sodium acetate during the process of separation and purification. Therefore, the term xe2x80x9cmixture comprising polytetrahydrofuran derivativesxe2x80x9d used herein, which can be depolymerized according to the invention, comprises one or more of the above components.
To carry out the depolymerization process, the PTMEG and/or PTMEA is mixed with the xcex2-zeolite and the mixture is heated at about 100-250xc2x0 C., and preferably at 130-210xc2x0 C. Those skilled in the art will be aware that if higher temperatures or higher concentrations of xcex2-zeolite are used, the reaction takes place more rapidly.